The present invention relates generally to arrangements for controlling a liquid fuel flow and more particularly to such arrangements employed in internal combustion engine carburetion systems with a view toward making such systems less susceptible to dirt or moisture blockage as well as compensating such systems for environmental temperature changes.
For exemplary purposes the present invention will be described in the environment of a diaphragm carburetor having an adjustable idle metering system and a fixed main metering system.
In present day diaphragm carburetors having adjustable idle and main metering systems, fuel will not flow through the main metering system until the air velocity through the venturi or carburetor throat is relatively high as caused by either high engine speed or loading the engine. This relatively high velocity and correspondingly low pressure required for main metering system fuel flow occurs because the pressure in the diaphragm cavity must be below atmospheric pressure by an amount sufficient to overcome the weight of the diaphragm assembly and the fuel resting on the diaphragm as well as the spring force exerted on the inlet needle valve for fuel to flow into the diaphragm cavity. This reduced pressure also affects the main discharge nozzle check valve so that a relatively high velocity of air movement through the carburetor venturi is required before the venturi pressure drop is sufficiently great to open the check valve allowing fuel flow to occur. Thus, unless the engine is run at a high rate of speed or subjected to a load, the main fuel nozzle may not be supplying fuel to the engine when adjustments to the main fuel nozzle needle valve are made, resulting in an improper richness setting of that needle valve and a reduction in overall performance of the engine.
The typical diaphragm carburetor also requires a check valve in the main metering system to prevent any air flow from the main discharge nozzle to the diaphragm cavity and into the idle metering system thereby causing the engine to stall when the speed is dropped to idle or causing the engine speed to be erratic if governed at moderate no load speeds. Such check valves are diposed in line with the valve seats with only a small portion of the valve or seat being washed by fuel flow through the assembly. Dirt, moisture and other deleterious materials can adhere to portions of the valve or seat not being cleansed by the fuel flow with such particles preventing the valve from closing when the engine speed is decreased to idle permitting air to backbleed into the idle system and causing the engine to stall.
Carburetor needle valves typically have a conically tipped adjusting needle which fits into a round orifice to form a metering cross section of generally annular configuration, the cross sectional area of which is controlled by the degree of penetration of the conical tip into the orifice. This small annulus tends to strain particles of a size smaller than those removed by the fuel system filter and has a large wetted surface area where ice can form restricting flow. Moisture accumulation and other surface tension problems may also restrict the fuel flow opening. As these particles are filtered at the annulus or as particles may accumulate downstream of the orifice due to turbulence, the area or effective fuel limiting dimension is reduced with a corresponding reduction in fuel flow causing the fuel-air ratio to become progressively more lean until engine performance becomes unacceptable. At this time, the mixture richness may be increased by withdrawing the conical adjusting needle tip from the orifice to bring the fuel limiting dimension or area back to a value to provide the correct fuel-air mixture ratio while at the same time increasing the particle limiting dimension between the orifice surface and the conical needle end surface, allowing the foreign material trapped at the annulus or the accumulation downstream to be flushed on through the system. Within a short period of time the fuel-air ratio now becomes too rich, again resulting in unacceptable engine performance and requiring a reinstatement of the original needle valve setting for optimum performance. One system for improving the ratio of minimum linear extent of the orifice to orifice area is to provide a tapered groove along the end portion of the adjusting needle.
Carburetion compensation to accommodate the fuel-air ratio to varying ambient conditions is known in a wide variety of forms including continuously variable automatic choke arrangements and operator accessible mixture controls. In small engine carburetor designs where simplicity and economy are paramount considerations, a rudimentary two or three position manual choke which is closed to start and opened to run, is generally the only control available to the operator. Such carburetors have in the past been set richer for winter starting and operation and more lean for normal summer starting and operation. Even seasonal equipment such as lawnmowers and snowthrowers are not used under constant environmental conditions. For example, a snowthrower might be used at ambient temperatures of 45.degree. F. in bright sunshine or at -20.degree. F. in the evening or on an overcast day. Optimum performance over such a relatively wide temperature range has been extremely difficult to achieve. Even with seasonally used equipment, the effects of temperature changes has made the manufacture or maintenance of such equipment during the off-season difficult. A significant cause of these problems is now believed to be the effect of temperature on the operation of the fuel metering system.